Throughout this application various publications are referred by arabic numerals to within parenthesis. Full bibliographic citations for these references may be found at the end of the specification immediately preceding the claims. The disclosures for these publications in their entireties are hereby incorporated by reference into this application to more fully describe the state of the art to which this invention pertains.
The c-kit proto-oncogene encodes a transmembrane tyrosine kinase receptor for an unidentified ligand and is a member of the colony stimulating factor-1 (CSF-1)--platelet-derived growth factor (PDGF)--kit receptor subfamily (7, 41, 57, 23). c-kit was recently shown to be allelic with the white-spotting (W) locus of the mouse (9, 17, 35). Mutations at the W locus affect proliferation and/or migration and differentiation of germ cells, pigment cells and distinct cell populations of the hematopoietic system during development and in adult life (47, 51). The effects on hematopoiesis are on the erythroid and mast cell lineages as well as on stem cells, resulting in a macrocytic anemia which is lethal for homozygotes of the most severe W alleles (46), and a complete absence of connective tissue and mucosal mast cells (72). W mutations exert their effects in a cell autonomous manner (28, 46), and in agreement with this property, c-kit RNA transcripts were shown to be expressed in targets of W mutations (35). High levels of c-kit RNA transcripts were found in primary bone marrow derived mast cells and mast cell lines. Somewhat lower levels were found in melanocytes and erythroid cell lines.
The identification of the ligand for c-kit is of great significance and interest because of the pleiotropic effects it might have on the different cell types which express c-kit and which are affected by W mutations in vivo. Important insight about cell types which may produce the c-kit ligand can be derived from the knowledge of the function of c-kit/W. The lack of mast cells both in the connective tissue and the gastrointestinal mucosa of W/W.sup.V mice indicated a function for c-kit in mast cell development. Mast cells derived from bone marrow (BMMC) are dependent on interleukin 3 (IL-3) and resemble mast cells found in the gastrointestinal mucosa (MMC) (92, 93). Connective tissue mast cells derived from the peritoneal cavity (CTMC) in vitro require both IL-3 and IL-4 for proliferation (79, 75). The interleukins IL-3 and IL-4 are well characterized hematopoietic growth factors which are produced by activated T-cells and by activated mast cells (92, 94, 95, 96, 97). An additional mast cell growth factor has been predicted which is produced by fibroblasts (47). In the absence of IL-3, BMMC and CTMC derived from the peritoneal cavity can be maintained by co-culture with 3T3 fibroblasts (98). However, BMMC from W/W.sup.V mice as well as mice homozygous for a number of other W alleles are unable to proliferate in the fibroblast co-culture system in the absence of IL-3 (99, 100, 38). This suggested a function for the c-kit receptor in mature mast cells and implied that the ligand of the c-kit receptor is produced by fibroblasts. Huff and coworkers recently reported the stimulation of mast cell colonies from lymph node cells of mice infected with the nematode Nippostronglyus brasiliensis by using concentrated conditioned medium from NIH 3T3 fibroblasts (84). A short term mast cell proliferation assay was developed which means to purify a fibroblast derived activity (designated KL) which, in the absence of IL-3, supports the proliferation of normal BMMC's and peritoneal mast cells, but not W/W.sup.V BMMC's. In addition, KL was shown to facilitate the formation of erythroid bursts (BFU-E). The biological properties of KL are in agreement with those expected of the c-kit ligand with regard to mast cell biology and aspects of erythropoiesis. The defect W mutations exert is cell autonomous; in agreement with this property, there is evidence for c-kit RNA expression in cellular targets of W mutations (35, 39). The recent characterization of the molecular lesions of several mutant alleles indicated that they are loss-of-function mutations that disrupt the normal activity or expression of the c-kit receptor (35, 100, 101, 36).
Mutations at the steel locus (Sl) on chromosome 10 of the mouse result in phenotypic characteristics that are very similar to those seen in mice carrying W mutations, i.e., they affect hematopoiesis, gametogenesis, and melanogenesis (5, 47, 51). Many alleles are known at the Sl locus; they are semidominant mutations, and the different alleles vary in their effects on the different cell lineages and their degree of severity (47, 51). The original Sl allele is a severe mutation. SIISI homozygotes are deficient in germ cells, are devoid of coat pigment, and die perinatally of macrocytic anemia (5, 50). Mice homozygous for the Sl allele, although viable, have severe macrocytic anemia, lack coat pigment, and are sterile. Both SII.sup.+ and Sl.sup.d /+ heterozygotes have a diluted coat color and a moderate macrocytic anemia but are fertile, although their gonads are reduced in size. In contrast to W mutations, Sl mutations are not cell autonomous and are thought to be caused by a defect in the micro-environment of the targets of these mutations (28, 30, 12). Because of the parallel and complementary characteristics of mice carrying Sl and W mutations, we and others had previously hypothesized that the Sl gene product is the ligand of the c-kit receptor (51, 9).
The proto-oncogene c-kit is the normal cellular counterpart of the oncogene v-kit of the HZ4--feline sarcoma virus (7). c-kit encodes a transmembrane tyrosine kinase receptor which is a member of the platelet derived growth factor receptor subfamily and is the gene product of the murine white spotting locus (9, 17, 23, 35, 41, 57). The demonstration of identity of c-kit with the w locus implies a function for the c-kit receptor system in various aspects of melanogenesis, gametogenesis and hematopoiesis during embryogenesis and in the adult animal (47,51). In agreement with these predicted functions c-kit mRNA is expressed in cellular targets of W mutations (3, 24, 25, 35, 39).
The ligand of the c-kit receptor, KL, has recently been identified and characterized, based on the known function of c-kit/W in mast cells (2, 14, 37, 38, 56, 58, 59). In agreement with the anticipated functions of the c-kit receptor in hematopoiesis KL stimulates the proliferation of bone marrow derived and connective tissue mast cells and in erythropoiesis, in combination with erythropoietin, KL promotes the formation of erythroid bursts (day 7-14 BFU-E). Furthermore, recent in vitro experiments with KL have demonstrated enhancement of the proliferation and differentiation of erythroid, myeloid and lymphoid progenitors when used in combination with erythropoietin, GM-CSF, G-CSF and IL-7 respectively suggesting that there is a role for the c-kit receptor system in progenitors of several hematopoietic cell lineages (27, 37).
Mutations at the steel locus on chromosome 10 of the mouse result in phenotypic characteristics that are very similar to those seen in mice carrying W mutations, i.e., they affect hematopoiesis, gametogenesis and melanogenesis (5, 47, 51). The ligand of the c-kit receptor, KL, was recently shown to be allelic with the murine steel locus based on the observation that KL sequences were found to be deleted in several severe Sl alleles (11, 38, 59). In agreement with the ligand receptor relationship between KL and c-kit, Sl mutations affect the same cellular targets as W mutations, however, in contrast to W mutations, Sl mutations are not cell autonomous and they affect the microenvironment of the c-kit receptor (12, 28, 30). Mutations at the steel locus are semidominant mutations and the different alleles vary in their effects on the different cell lineages and their degree of severity (47, 51). The original Sl allele is an example of a severe Sl mutation. Sl/Sl homozygotes are deficient in germ cells, are devoid of coat pigment and they die perinatally of macrocytic anemia (5,50). Mice homozygous for the Sl.sup.d allele, although viable, have severe macrocytic anemia, lack coat pigment and are sterile (6). Both Sl/+ and Sl.sup.d /+ heterozygotes have a diluted coat color and a moderate macrocytic anemia, but they are fertile, although their gonads are reduced in size. Southern blot analysis of Sld/+ DNA by using a KL cDNA as a probe indicated an EcoR1 polymorphism, suggesting that this mutation results from a deletion, point mutation or DNA rearrangement of the KL gene (11).